The invention relates to a method of suppressing unwanted indications in ultrasonic testing in automated systems.
It has been known that it is possible in automated ultrasonic testing to distinguish unwanted indications over actual flaw indications by a plurality of techniques. Among the known methods are the following:
(a) Statistical Interference Suppression
By counting the amplitude-discriminated signals, which are above the so-called monitor thresholds or evaluation thresholds, in uninterrupted sequence and preselected numbers.
(b) Extended Statistical Interference Suppression
By counting the amplitude-discriminated signals, which are above a low evaluation threshold, in uninterrupted sequence and preselected numbers, while at the same time a higher evaluation threshold is exceeded at least once.
(c) Aerial-Type Suppression
By means of additional amplifiers, signals (normally spurious electrical signals) are picked up from the air via an aerial and are amplified. The amplified signals inhibit the so-called monitors provided for test evaluation in dependence on the propagation times typical for the ultrasonic amplifiers.
However, all of the above-mentioned techniques have drawbacks when, for instance, upon use of the statistical interference suppression (which requires that the monitor or evaluation threshold is exceeded during several test cycles in succession) or upon use of the extended statistical interference suppression (which requires that the fixed lower monitor threshold is exceeded during several test cycles in successsion and that the likewise fixed upper monitor threshold is exceeded in at least one test cycle) the flaw signal is either below the preset value of the number of successively required flaw signals or in the case of aerial-type suppression the necessary conditions are not met by electrical signals.
Indeed, in the case of extended statistical interference suppression there is a considerable limitation at relatively high amplitudes due to the lower discriminator threshold which is fixed prior to commencement of the test. In case of a fixed discriminator threshold the signal dynamics expected due to the test cannot be taken into account. The mentioned prior art is substantially described in German patent DE-PS 2,704,132.
It is the object of the present invention to provide a method of blanking unwanted indications, by which the deficiencies in prior art methods are eliminated and the ultrasonic testing becomes more reliable.